If you need help coding CSS, maybe you could ask someone on the forum. If you have a particular background you'd like to use, you could upload it to Photobucket.
You could find them anywhere. You just have to know how to code them properly, however. I wiped my entire CSS and added some stuff manually using the Layout/HTML page. The CSS is quite confusing to me, and if you don't know HTML, you definitely won't understand CSS. I guess I haven't taken the time to learn CSS.
If by bg you mean background..there are some already on the site..but if you don't like them, then I don't know what to tell you. If you didn't already know...to get to them you go to your my mibba then on the sidebar that says profile there will be a tab that says layout/HTML that's where they are.